The present invention relates to an X-ray image detector for converting an image which is obtained from an X-ray being a radiation to an optical or electric image signal.
Generally, an X-ray is useful in the examination of an internal structure of a human body or an object and the apparatus for converting a penetration density distribution of an X-ray illuminating the human body and object, that is, the X-ray intensity distribution or X-ray image, to a visible light image or electric image signal corresponding to the X-ray have been extensively utilized.
As the apparatus for converting the X-ray image to a visible light image or electric image signal, an X-ray imaging tube (X-ray image intensifying tube) and X-ray vidicon (X-ray image detector) have been developed.
The X-ray imaging tube for intensifying an X-ray image signal and converting it to a visible light image comprises a vacuum container holding a vacuum therein, an input fluorescent screen arranged in the vacuum container and converting an X-ray which is incident from an outside to fluorescent light, a photoelectric screen arranged in the vacuum container and converting the fluorescent light which is exited from the input fluorescent screen to a photoelectron, an output fluorescent screen arranged in the vacuum container and converting, to a visible light image, the photoelectron from the photoelectric screen which is accelerated in response to an electric field provided by an acceleration electrode provided on an inner side, a metal layer stacked on the output fluorescent screen, and an output window holding the output fluorescent screen and through which the visible light image passes.
The above-mentioned X-ray imaging tube once converts an X-ray image which is incident on the vacuum container to a visible light image by the input fluorescent screen and, while accelerating a photo-electron by an electron lens after the visible light image has been converted to that photoelectron, reduces the size of the image to intensify it to a high energy and converts it once more back to a visible light image by the output fluorescent screen to obtain a brighter visible light image than the image obtained at the input fluorescent screen.
That is, the visible light image obtained by the photoelectric conversion of the X-ray image incident on the input fluorescent screen is very weak in intensity. By accelerating the photoelectron under an electric field from an acceleration electrode after the visible light image has been converted by a photoelectric screen to that photoelectron and, by doing so, intensifying the energy of the photoelectron to obtain a higher energy than an energy of an original X-ray image and, thereafter, once again converting it back to visible light by the output fluorescent screen, it is possible to obtain a more intensified visible light image than the original X-ray image.
In these days, in order to reduce the size of the apparatus, it has been proposed that an output section be made flat-like or an X-ray imaging tube be made to have features at the output section. For example, it has been proposed in U.S. Pat. No. 4,300,046 that an output fluorescent screen stacked over a transparent glass substrate be arranged parallel to an input fluorescent screen at a given spacing. It is to be noted that, in this structure, a proximity type electron lens is used.
In the structure disclosed in U.S. Pat. No. 4,300,046, the output fluorescent screen is formed integral with the glass substrate of a vacuum container and it is possible to directly acquire a visible light image from an outside without requiring an output window. Further, an X-ray image converted to a photo-electron is guided directly to the output fluorescent screen, so that the distortion of the image is reduced.
In JPN PAT APPLN KOKAI PUBLICATION NO. 61-62283, an X-ray image detector is disclosed having a circuit comprising thin-film photodiodes and thin-film transistors (TFTS) formed at a glass substrate and a fluorescent screen stacked over the circuit.
This X-ray image detector converts an incident X-ray image to light by a fluorescent screen, converts the light to an electric signal by the photodiode and takes the electric signal as image information to an outside by the TFT.
On the other hand, an example in which, in order to improve an S/N (signal to noise) ratio of an output image, a microchannel plate (MPC) for multiplying an electron is arranged in the X-ray imaging tube is suggested in U.S. Pat. No. 3,394,261.
The above-mentioned X-ray imaging tube can multiply a very weak X-ray image as an easily observable visible image and, since an electron lens is used to multiply (intensify) an X-ray image converted to a photoelectron, there arises a problem of an image distortion resulting from the electron lens. Further, a greater space is required for the electron lens and, as a result, the size of the X-ray imaging tube becomes greater, presenting a problem.
In the apparatus disclosed in U.S. Pat. No. 4,300,046, a proximity type electron lens is used and a smaller space is required than a structure using an ordinary electron lens, so that the size of the X-ray imaging tube can be made smaller. Further, it is recognized that there is less image distortion resulting from the. characteristic of the proximity type electron lens. Since, however, the proximity type electron lens cannot vary the size of an X-ray image converted to a photoelectron, it is difficult to, like the ordinary X-ray imaging tube as set out above, reduce the size of an electron image and, by doing so, enhance the energy density and improve the light strength per unit area of an output image. In comparison with the ordinary type X-ray imaging tube as set out above, the image luminance of an output is reduced to about 1/10 and, for use in a medical field, an X-ray amount with which the human body has to be irradiated is increased, thus presenting a problem of increasing an amount of X-ray exposed to the human body.
In the apparatus disclosed in JPN PAT APPLN KOKAI PUBLICATION NO. 61-62283, in spite of the fact that an X-ray image converted to an electric signal is a signal which is very weak in intensity, it is not multiplied until it is externally taken out and there arises a problem that more noise emerges on the output image. Although the X-ray image detector of a structure where an amplifying function is added in a TFT circuit has been proposed in JPN APT APPLN KOKAI PUBLICATION NO. 5-130510, etc., it is difficult to uniform the multiplication factor of the multiplying element given to all the pixels. This presents a problem of never improving an image quality over a whole image region.
It is possible, as reported in U.S. Pat. No. 3,394,261, to increase an image luminance of an X-ray image output by setting the MCP in the X-ray imaging tube, but in actual practice it is difficult to increase the size of the MCP. In the case where no consideration is paid to restricting the size of the X-ray imaging tube, it is possible to construct the X-ray imaging tube by increasing the magnifying power of the electron lens and to do so with the use of a smaller MCP. However, a larger space is required for the electron lens, thus offering a problem not suited to a practical application.
It is accordingly the object of the present invention to provide an X-ray image detector which can reduce the size of a whole apparatus associated with an X-ray imaging tube, reduce noise components of an output X-ray image even if an incident X-ray is very weak and obtain an output image of an X-ray image of a larger area not involving an image distortion containing an adverse effect resulting from a moire, etc.
The present invention is achieved based on the above-mentioned problem and provides an X-ray image detector comprising a vacuum container holding a vacuum therein; an input fluorescent screen arranged in the vacuum container and converting an X-ray which is incident from an outside to fluorescent light; an output fluorescent screen arranged in the vacuum container and generating a visible light image by an electron accelerated under an electric field; a metal layer stacked on the output fluorescent screen; an output window holding the output fluorescent screen and allowing the visible light image to pass therethrough; and wherein the input fluorescent screen and output fluorescent screen are arranged parallel to each other while maintaining a predetermined spacing; an electron multiplier is arranged between the input fluorescent screen and the output fluorescent screen, the electron multiplier has a plurality of metal plates of a predetermined thickness having a plurality of through holes opened and a plurality of insulating materials having gaps to allow a photoelectron from the through hole to pass therethrough are alternately stacked; and image pick-up element is arranged in a position allowing a visible light image from the output fluorescent screen to be received past the output window and takes, as a video image, image information which is output to the output window.